The ER chaperone, BIP protects Microglial cells from ER stress-mediated Apoptosis in Hyperglycemia

Abstract

Background: Binding of Immunoglobulin heavy chain protein(BIP) is a major endoplasmic reticulum (ER) chaperone facilitating the assembly of newly synthesized proteins in the ER. Microglial cells vigorously respond to brain injuries and eliminate the damaged neuronal and apoptotic cells through phagocytosis in the central nervous system. However, the mechanism of BIP-mediated microglial cell function is not clear in hyperglycemia. We explored the molecular mechanism of BIP in microglial function during hyperglycemia conditions. Methods: Hyperglycemia was induced in C57BL/6J mice by two consecutive intraperitoneal injections of streptozotocin (STZ 100/kg) and confirmed by measuring the blood glucose from day 2 to day 14. After 14 days of experimental condition, mice were sacrificed, brains were collected, and tissue lysate was prepared for ER chaperone studies. In-vitro hyperglycemia was induced by exposing HMC3 cells to 25mM glucose for 5 days and proteins involved in ER stress, apoptosis, and autophagy were analyzed. For the BIP induction, cells were treated with 25μM of BIX (BiP inducer-X) after 48 hr of hyperglycemia for 3 days. Results: In hyperglycemia condition, the major ER chaperone BIP protein expression was dramatically reduced in HMC3 cells, which led to increased apoptosis through the activation of CHOP and mitochondrial pro-apoptotic proteins (Bax, Bad, cleaved caspase-3). The flow cytometry results also indicate that hyperglycemia-induced the apoptosis and reactive oxygen species (ROS) production. Interestingly, the BIP inducer BIX restored the apoptosis in microglia cells through the derepression of BIP expression and inhibition of ER stress. Conclusion: These results suggest that the ER chaperone BIP is required for the microglial function and protection from apoptosis in hyperglycemia. A better understanding of the molecular mechanism and role of BIP in microglia function may contribute to the development of novel therapies for microglia dysfunction-associated neurodegenerative diseases.